Photodynamic therapy, hereinafter also referred to as “PDT”, is a process for treating cancer wherein visible light is used to activate a substance, such as a dye or drug, which then attacks the tumor tissue through one or more photochemical reactions, thereby producing a cell-killing, or cytotoxic, effect. When certain photosensitizer compounds are applied to a human or animal body, they are selectively retained by cancerous tissue while being eliminated by healthy tissue. The tumor or cancerous tissue containing the photosensitizer can then be exposed to therapeutic light of an appropriate wavelength and at a specific intensity for activation. The light energy and the photosensitizer cause a photochemical reaction which kills the cells in which the photosensitizer resides.
Phthalocyanines, hereinafter also abbreviated as “Pcs”, are a group of photosensitizer compounds having the phthalocyanine ring system. Phthalocyanines are azaporphyrins consisting of four benzoindole groups connected by nitrogen bridges in a 16-membered ring of alternating carbon and nitrogen atoms (i.e., C32H16N8) which form stable chelates with metal and metalloid cations. In these compounds, the ring center is occupied by a metal ion (either a diamagnetic or a paramagnetic ion) that may, depending on the ion, carry one or two ligands. In addition, the ring periphery may be either unsubstituted or substituted. The synthesis and use of a wide variety of phthalocyanines in photodynamic therapy is described in International Publication WO 2005/099689. Phthalocyanines strongly absorb clinically useful red or near IR radiation with absorption peaks falling between about 600 and 810 nm, which potentially allows deep tissue penetration by the light.